This invention relates to a coaxial vacuum feedthrough that is used for connecting an external wire to electric equipment inside a vacuum vessel. More particularly, it relates to a shield-type coaxial vacuum feedthrough that permits delivering high-frequency signals, such as microwaves, to electric equipment inside a vacuum vessel such as an accelerator.
Generally, a coaxial feedthrough is used to send high-frequency current into and out of electric equipment. A conventional general-purpose coaxial feedthrough of this type comprises a pin of metal in the shape of a round bar which is enclosed with a shielding cylinder of synthetic resin such as ethylene tetrafluoride, then enclosed with a metal cylinder. In baking a vacuum vessel, for the purpose of evacuation, fitted with such a coaxial feedthrough, a noxious gas may develop, and flow into the vessel, as the vessel wall becomes heated.
This problem may appear to be solved by changing the material of the shielding cylinder from synthetic resin to ceramic, while leaving the metal round-bar pin and cylinder unchanged. But, in reality, this method changes the characteristic impedance of the feedthrough because of the difference in dielectric constant between synthetic resin and ceramic. Then the characteristic impedance of the feedthrough cannot be matched with that of a coaxial cable that is to be coupled therewith, especially in the high-frequency region.